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Posted: 8/9/2026 9:03:55 AM EDT
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First caveat: Ammunition price and availability is not a concern. Second caveat: This is fun gun, not a life depends on it piece (until the concept is proven reliable) Basic build: Aero enhanced quad rail with notched barrel nut (you will see why later) with custom extended heat sink A3/4 upper (depended on the barrel) Adams Arms Rifle length piston system with 3 position railed gas block F/A chrome lined carrier STD lower with A2 buffer system and FRT Barrel options: Armalite 19" chrome-lined 5.56 NATO mid weight barrel with 1/9 twist. Honestly one of the most accurate factory barrels I have ever owned which is sub MOA with my 75 grain handloads. OR WOA 18" stainless steel 6.8 SPC mid weight barrel with 1/10 twist and spec 2 chamber. This barrel shoots MOA all day long, sub MOA when I shoot better. Leaning towards the Armalite because of the chrome lining and chamber, but I feel it's accuracy would be greatly wasted and will better served in DMR role. My reasoning for the 6.8 SPC is primarily being able to send more weight down range, plus the barrel is not being used for it's original purpose. |
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6.8 limits your magazine capacity too much. I would go 5.56 so you have access to 60rd mags. I like my 6.8 with FRT, but mine is a LWRC Six8 which at least gets me to 30rds. Mine is really short and 6.8 does well from 8" barrels. FRT and auto setups burn ammunition really quickly. 30rd mags are on the small side with them. |
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Its up to you of course but I think 6.8 is a dead/dying cartridge. If you want more power 4jump to 308. I think the IAR is a cool project. Ive dome one in a 24" HEAVY match barrel (in retrospect its too long). I did another with a 20" HBAR barrel about right, and for fun one with a 12" HBAR and a PDW brace. The 12" is my favorite They all have LMG style bipods and the 12" has a front grip. The 20 inch and the 24 inch have KNS adapters to run on MG3 AA tripods. I have a Fightlite beltfed upper and just picked up one of the lowers without the magwell to make the beltbag sit higher. Its a pretty cool set up but it has a few drawbacks. The IAR/AR-RPK are more easily carried and run plus they used standard parts The 24 and 20 guns have ELCANS and IR lasers. The 12" has a Red Dot, Im tempted to add a magnifier but really see it as a close in gun. Ive got a stack of 40 round mags because they are easier to carry then the drums. I picked up one of the 100 round BETA drums (clones) but havent tested it yet Ive been tempted to do a 308 version but havent gotten there yet |
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Originally Posted By KitBuilder: Chrome lined 5.56mm YES and an upgraded gas tube for better heat dispersion Check out what Red Right Hand does. The Colt Diamco LMG uses a MUCH thicker and straight gas tube. Thats not an easy redo for civilians but Red Right Hand does a "heat sink" to fix the issue. Its NOT cheap but if you really want to use it like a IAR/AR-RPK especially with a FRT or even a Binary then the gas tube will be your issue. I suppose you could try a Harry Homeowner method of wrapping the factory gas tube with copper wire in coils as a crude heat sink but you still have the bend which is where the heat is most likely to rupture the tube |
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Originally Posted By KitBuilder: Chrome lined 5.56mm For this role, that makes the most sense, by far. OP states ammo cost is not a concern. If you want to get good with an 'auto' rifle, ammo cost WILL be a concern. Same with an MG, range toy or not. |
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| You also need to consider OPTICS. Im a fan of the old ELCAN but its big and heavy. I suppose a ACOG would work but I dont like anything much more than 4x. It needs to be fast and uncluttered. The Brits used a SUSAT on their L1A1s and Enfield L85s, they had a pointer coming from the top which kept the important part of the FOV clear. |
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Thanks for the replies! Going the 5.56 route, it just makes more sense: Higher capacity feeding devices Commonality of parts, have a feeling a 6.8 bolt will not last as long in a IAR Factory ammo is cheaper (if this proves reliable for me and want to take it to the take step) Chrome lining of the bore and chamber As for optics, I will be going with a FFP LVPO and use it like a poorman's Elcan Specter DR. |
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Originally Posted By 223Rem: Thanks for the replies! Going the 5.56 route, it just makes more sense: As for optics, I will be going with a FFP LVPO and use it like a poorman's Elcan Specter DR. Your choice but Id look at something other than a LVPO. Even on 1X they are slow. You dont want slow for a LMG. Id look at something like a Eotech and if you insist on magnification then add a clip on BUT if youre using the gun as a LMG then you wont want magnification. A BIG and Fast optic will let it run like a LMG anything else is just a heavy rifle that wants to be a skinny girl but wants to eat ice cream and lay on the sofa UP to you |
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Originally Posted By 223Rem: Thanks for the replies! Going the 5.56 route, it just makes more sense: Higher capacity feeding devices Commonality of parts, have a feeling a 6.8 bolt will not last as long in a IAR Factory ammo is cheaper (if this proves reliable for me and want to take it to the take step) Chrome lining of the bore and chamber As for optics, I will be going with a FFP LVPO and use it like a poorman's Elcan Specter DR. Right choice. We also have an ongoing thread on this that may give you some ideas. https://www.ar15.com/forums/AR-15/AR-RPK-Build-Pictures-Ideas-Part-II/118-789452/ |
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Originally Posted By BrandonP: Why not the actual IAR upper? Arms Unlimited had the 16" uppers for sale recently. They're perfect for what you want to do - massive heat sink and chrome lined barrel. Because I am not that rich? Plus, I like building the uppers myself. |
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Have a 6.8 SPC2 and its a great mid range cartridge for hunting. set up with 12.5 barrel, using PRI 6.8mm mags. This is great for hogs, yotes, deer, and out to max 400 yds. Not something I would mag dump, but for a backup hunting rifle or fun hunting rifle sub 400, it is a blast. |
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Originally Posted By 223Rem: First caveat: Ammunition price and availability is not a concern. Originally Posted By 223Rem: Because I am not that rich? Plus, I like building the uppers myself.  You aren't rich enough for a $1400 upper but were considering 6.8SPC at full auto rates of fire? |
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Originally Posted By Gotterdammerung: https://media1.giphy.com/media/UvwI1X7XkbXq0/200.webp?cid=8dc8958cs0xffn48vmdyry6wyoz5qtb8yrhhc5ibhxks2vg0&ep=v1_gifs_search&rid=200.webp&ct=g You aren't rich enough for a $1400 upper but were considering 6.8SPC at full auto rates of fire? This, if the goal is to hang it on the wall and tell people about it just say so. Otherwise selling some junk and a little bit of that unlimited ammo stash could get you $1400 easy |
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Originally Posted By Gotterdammerung: https://media1.giphy.com/media/UvwI1X7XkbXq0/200.webp?cid=8dc8958cs0xffn48vmdyry6wyoz5qtb8yrhhc5ibhxks2vg0&ep=v1_gifs_search&rid=200.webp&ct=g You aren't rich enough for a $1400 upper but were considering 6.8SPC at full auto rates of fire? Maybe because I have the mean to produce 6.8 SPC rounds for about $0.25 each? Cheaper if I use surplus powder, but I do not get the MV I prefer. So for the $1400, I can make 5600 rounds for 6.. Beside by using an FRT, I DO NOT have to shoot F/A all the time. Secondly, I would prefer a longer barrel without a DI gas system. Trying to keep as much heat and gas out of the action as possible. Plus I want to give it a cool name like the M.250 or the SLMG 6.8. |
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